X-ray computed tomography (CT) scanners have been used in security screening in airports for several years. A conventional system comprises an X-ray tube that is rotated about an axis with an arcuate X-ray detector also rotated at the same speed around the same axis. The conveyor belt on which the baggage is carried is placed within a suitable aperture around the central axis of rotation, and moved along the axis as the tube is rotated. A fan-beam of X-radiation passes from the source through the object to be inspected to the X-ray detector array.
The X-ray detector array records the intensity of X-rays passed through the object to be inspected at several locations along its length. One set of projection data is recorded at each of a number of source angles. From these recorded X-ray intensities, it is possible to form a tomographic (cross-sectional) image, typically by means of a filtered back projection algorithm. In order to produce an accurate tomographic image of an object, such as a bag or package, it can be shown that there is a requirement that the X-ray source pass through every plane through the object. In the arrangement described above, this is achieved by the rotational scanning of the X-ray source, and the longitudinal motion of the conveyor on which the object is carried.
In this type of system, the rate at which X-ray tomographic scans can be collected is dependent on the speed of rotation of the gantry that holds the X-ray source and detector array. In a modern CT gantry, the entire tube-detector assembly and gantry will complete two to four revolutions per second. This allows up to four or eight tomographic scans to be collected per second respectively.
As the state-of-the-art has developed, the single ring of X-ray detectors has been replaced by multiple rings of detectors. This allows many slices (typically 8) to be scanned simultaneously and reconstructed using filtered back projection methods adapted from the single scan machines. With a continuous movement of the conveyor through the imaging system, the source describes a helical scanning motion about the object. This allows a more sophisticated cone-beam image reconstruction method to be applied that can in principle offer a more accurate volume image reconstruction.
In a further development, swept electron beam scanners have been demonstrated in medical applications whereby the mechanical scanning motion of the X-ray source and detectors is eliminated, being replaced by a continuous ring (or rings) of X-ray detectors that surround the object under inspection with a moving X-ray source being generated as a result of sweeping an electron beam around an arcuate anode. This allows images to be obtained more rapidly than in conventional scanners. However, because the electron source lies on the axis of rotation, such swept beam scanners are not compatible with conveyor systems which themselves pass close, and parallel, to the axis of rotation.